Radzievsky , G. V. “Equivalence of Almost Root Vectors of Polynomial Beams of Operators ”. Ukrains’kyi Matematychnyi Zhurnal 44, no. 7 (August 3, 1992): 948–970. Accessed April 14, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/8128.